Recently, frozen dough for bread has become very attractive to the baking industry because it has the following great advantages; i.e., (1) it may be useful in the supply of fresh-baked bread; and (2) it has vast merits for solving industrial productivity problems by shortening the baking process (savings of labor) and reducing the need for a night shift. Frozen dough is produced by kneading and fermenting materials for bread, keeping the material under freezing at around -20.degree. C. The frozen dough is subjected to baking after, if necessary, proofing. Common bakers' yeast is so likely damaged by fermentation prior to freezing that the use of yeast is limited to a case where the dough is enriched with a relatively large amount of materials such as sugar, fat, egg, milk products, etc. is not subjected to fermentation or subjected to fermentation for only a short time prior to freezing. When the dough containing common bakers' yeast which has been fermented for a short time before freezing is thawed and baked immediately after proofing, it cannot be sufficiently baked so that it causes some problems such that flavor and taste of bread may be deteriorated. A process wherein the frozen dough is thawed and subjected to proofing requires a long time for baking, so that the purpose of frozen dough method may be lost.
Thus, there has been a demand for a kind of bakers' yeast which has a great freeze-resistance, may be hardly damaged by storage under the frozen condition after fermentation, during preparation of frozen dough. Some reports have presented kinds of yeast having freeze-resistance, i.e., Saccharomyces rosei (Japanese Patent Kokoku No. 59-25584), Saccharomyces cerevisiae FTY (Japanese Patent Kokoku No. 59-48607), Saccharomyces cerevisiae IAM4274 (Japanese Patent Kokai No. 59-203442). Both Saccharomyces rosei and Saccharomyces cerevisiae FTY (FRI-413) do not have strong maltose-fermentative ability and therefore, they are not suitable for use in frozen dough with the sugar content in the range of 0 to 20% by weight based on flour, i.e., from non-sugar dough to dough with a moderate sugar level. Saccharomyces cerevisiae IAM4274 has maltose-fermentative ability but does not show sufficient freeze-resistance in the dough with the sugar content in the range of 0 to 20% by weight based on flour, i.e., from non-sugar dough, etc. to the dough with a moderate sugar level. Size of the yeast Saccharomyces rosei is smaller than that of common bakers' yeasts, so that it takes a long time for separation, washing and dehydration of yeasts. The other reports have presented bakers' yeast which is suitable for the dough for lean type bread and has maltose-fermentative ability and freeze-resistance, i.e. Saccharomyces cerevisiae KYF110 (Japanese Patent Kokai No. 62-208273), and a fusant strain Saccharomyces cerevisiae 3-2-6D (Japanese Patent Kokai No. 63-294778) which is obtained by the cell fusion technique and has strong maltose-fermentative ability and enhanced freeze-resistance. However, these are not satisfactory yet.
As heretofore mentioned, there has not been obtained bakers' yeast which has strong fermentative ability and great freeze-resistance of non-sugar dough up to the dough with a moderate sugar level (up to 20% by weight based on flour), and which is appropriate for the non-sugar dough without sugar such as French bread and bread crumb or the dough for a white bread. Therefore, it has been extremely difficult to produce frozen dough for such kinds of bread.
The common bakers' yeast commercially available for non-sugar dough has a drawback on storability because of the rapid decrease of its fermentative ability under storage in the form of products, compared with the deterioration rate of ordinary bakers' yeasts having high fermentative ability of dough of a higher sugar level and middle fermentative ability of non-sugar dough, which are widely used for the bread with the sugar content in the range of 5% to 30% by weight based on flour.